1. Field of the Invention
The invention relates to transformers with an electrical insulation for the windings thereof constructed from copper sections, of solid and liquid insulating materials which together are effective in insulating the copper sections, the liquid insulating materials of which serves at the same time as the cooling liquid.
2. Description of the Prior Art
Tests on transformers with windings of copper sections have shown that metallic copper as well as copper which is dissolved in the oil are catalysts for the thermally oxidative aging of oil and cellulose. Through this oxidative aging process, the service life of transformers, especially of such transformers having a large amount of dissipation, is limited due to aging.
In addition, the copper which is dissolved and oxidized during the transformer operation through the influence of aging products of the oil and the cellulose can be deposited under reducing conditions as predominantly metallic copper on insulating parts, weakening or even damaging the insulation.
One possibility for preventing copper from the winding and its leads from being dissolved by transformer oil consists of covering the exposed copper surfaces by varnishing. While this method is used, it is expensive from a manufacturing point of view. A further possibility for covering the copper surfaces consists of nickel-plating them. This method, however, is less economical than varnishing the exposed copper surfaces.
Another method for preventing oxidation and dissolution of copper by the transformer oil is to add stabilizing additives to the transformer oil. An additive which is suitable for this purpose is, for instance, 2,6-di-tert.butyl-p-cresol which, when dissolved in the oil, acts as an oxidation inhibitor. It is a disadvantage of this solution to the oxidation problem, however, that such oxidation inhibitors must be completely dissolved in the transformer oil in relatively large amounts to be effective.
The dissolution and absorption of copper by transformer oil can also be prevented in principle by employing catalytically less active aluminum as the material for the winding conductors, instead of copper. However, the use of aluminum as an alternative to copper in windings of large transformers has not found acceptance so far, since with the existing price relationship to copper, aluminum does not offer enough advantages because of its lower electric conductivity, its less advantageous mechanical properties and its poorer workability.